Even though we sometimes hear about high-profile spills on the news, the mishandling of hazardous materials and waste isn't a rare occurrence. According to the Environmental Protection Agency (EPA), it has happened in tens of thousands of sites across the United States, where the hazardous materials have contaminated land, water and/or air [source: EPA]. Materials known as adsorbents are essential in the cleanup process.
It's important to note here that adsorption is not the same process as absorption. An absorbent sucks up and contains other materials into its body, causing the volume of the absorbent to swell up. Think of a sponge or a paper towel that soaks up your spilt milk. On the other hand, an adsorbent attracts other liquids or gases onto its surface only. However, the "surface" can also include internal surfaces, like pores and capillaries. Highly porous substances, like activated carbon, are great adsorbents.
An adsorbent bond can be either physical or chemical. Physical adsorption involves weak electrostatic attraction (also called van der Waals attraction) between molecules. Polar substances that have negative and positive surface charges, like water, are attracted to polar adsorbents. Non-polar substances with no distinct surface charge, like organic substances, are attracted to non-polar adsorbents. Physical adsorption is reversible in a process called desorption. Chemical adsorption, however, happens as a result of a chemical bond and is largely irreversible. Because adsorbents are reusable in the physical adsorption process, they're cheaper and preferable for environmental cleanup purposes.
The process for adsorption often involves what's called a fixed bed adsorber, in which a substance such as air passes through a bed of solid adsorbent. As the air passes, the adsorbent attracts the unwanted particles contained in the air. Placing multiple layers of these beds helps improve efficiency. This can be a regenerative process, where the adsorbent can go through desorption and then be put back into service.
Now that we know a little about what adsorption is and how it works, we can understand how adsorbents are useful for cleaning hazardous material. Read more on the next page.